After implementing the Schlieffen plan to good effect, the German army advanced deep into French territory, forcing the French armies to pull back. With the Germans dangerously close to Paris, Generalissimo Joffre gave the order to mount an offensive. The ensuing battle, which raged from 6th to 9th September 1914, ended with the retreat of the Germans, signifying the complete abandonment of the original war plans.

The Battle of the Marne

After advancing rapidly across Belgium, the German army defeated the French in the double battle of the borders. Despite the success of Lanrezac's 5th army during the battle of Guise on 29 September, Generalissimo Joffre gave the order, on 30 August 1914, for a general retreat to the Seine, allowing the German armies to advance on Paris.

In the capital, false news reports were the order of the day, the intention being to keep the people's morale high, even as the enemy's troops were stationed just a few dozen kilometres outside the city. As late as 23 August, it was still being reported that the French were holding Mulhouse, and that Anvers was still holding out.

Though the people's spirits may have been boosted by the false optimism of the newspaper reports, the military leaders, who, for their part, were well aware how dire the situation was, took drastic measures. On 26 August, General Gallieni was appointed military governor of Paris. He was tasked with protecting the capital, whilst the government left for Bordeaux the next day. There was a change of atmosphere. A communiqué released on 25 August reported that the French had retreated; the very next day, the newspapers announced that Paris was under threat. Consequently, some half a million people followed the example set by the government and fled the capital. As a result, 30 August and 2 September, only about 62% of the population remained in the city.

On 1 September back on the frontlines, the right flank of the German army, led by von Kluck, finding its numbers depleted, abandoned the original plan of marching on Paris, and veered eastwards, so as to cut off the Allies' retreat. By doing so, it moved away from von Bülow’s 2nd German army, which was advancing more slowly on the eastern flank. Meanwhile, Generalissimo Joffre opted to retreat, in order to regroup as best as he could, and take up a more advantageous position. However, when he learned of the German retreat, he decided to act. In spite of the differences of opinion among the Allies, Joffre managed to convince field-marshal French, the commander of the British Expeditionary Force (BEF) (who was initially reluctant to countenance a new operation and urged a retreat behind the Marne), to engage his units alongside the 5th French army. On 4 September, Joffre signed his famous order of the day, which decreed an about-turn. On paper, the two sides seemed fairly evenly matched, with 81 Allied divisions ranged against 80 German divisions. On the ground, however, the Allies had one million men at their disposal, compared to 750,000 on the German side. In the area where the main thrust of the attack was to be focused, between the Marne valley and Verdun, the French and British forces, with 66 divisions against 51, had a clear numerical advantage.

The order to attack was given on the morning of 5 September. Maunoury's army, with support from units led by Gallieni, launched an attack against von Kluck's right flank, in the direction of the Ourcq. On 6 September, the offensive began all along the front line, from the Ourcq to Verdun. As Gallieni requisitioned some 700 Parisian taxis to take infantrymen from the capital to the front line, Moltke began to worry. It was the first time the French had won a victory against the Germans in an open battle. On 7 September, von Kluck retreated to an area north of the Marne, and von Bülow followed suit, thus leaving a breach of about 35km in the German lines. The BEF and Franchet d'Espèrey's 5th army were quick to take advantage of the situation. The cavalry made a significant breakthrough and forced the Germans to retreat still further. The battle was joined along a 105km front, and there were heavy casualties on both sides. On 9 September, the outcome was still uncertain, and Joffre ordered Sarrail, whose army was ranged against that of the Crown Prince, near Verdun, to abandon his position should the need arise; but the latter, who was short of reserves, failed to assert himself. On 10 September, the Allies enjoyed successes in every sector: Manoury, in the north-east of Paris; French and Franchet d'Espèrey on the Deux-Morins, then on the Ourcq, in the breach between von Kluck and Bülow; Foch in the marshes of Saint-Gond; de Langle at Vitry-le-François, and Sarrail in Argonne. Acting on information provided in a telegram by lieutenant-colonel Hentsch, Moltke ordered a retreat from Nancy to the Vesle. This retreat marked, not only the end of the battle of the Marne, but also the final abandonment of the original plans on both sides: the Schlieffen plan on the German side, and the plan of the 17th French army on the Allied side.

The causes of this 'miracle' of the Marne have long been the subject of debate among historians, who have put forward differing views about which particular individual or factor played a decisive role. Pierre Renouvin has written that the Marne was "a victory for proficient high command". The German side did indeed suffer from a lack of coordination in the movements of its two armies, but also from the fact that its High Command was too far away from the theatre of operations. Stationed in Luxembourg, without access to telephones (the Germans were not using wireless telegrams), Moltke had difficulty obtaining precise intelligence. Among the major factors in the success of the French and British sides, those of particular note are, on the one hand, the numerical advantage of the Allies in the key sectors, due to the fact that two German armies were urgently sent back to the eastern front, and British troops entered the battle; and, on the other hand, a better logistical system for the management of food supplies: the Allies, with their backs to the capital, managed to take advantage of the dense network of railways which was already in place, whereas the Germans, whose supply lines came from Brussels, suffered as a result of gaps in their logistical chains.